Process of preparing chromium oxide gel catalysts



Patented June 30,1942

PROCESS OF PREPARING CHROMIUM OXIDE GEL CATALYSTS Glen H. Morey,

Terre Ilaute, lnd., assignor to Phillips Petroleum Company,

a corporation of Delaware No Drawing. Application November 27, 1936,Serial No. 113,091 23 Claims. (Cl. 252-254) This invention relates to ofpreparing catalysts. and it has particular relation to improvements in aprocess of preparing catalysts of gel characteristics and containingchromium oxide as a constituent of the gel.

Catalysts which contain chromium oxide and which possess gelcharacteristics have been used industrially to facilitate the conversionof various raw materials into more useful or economically more desirablematerials. as by the hydrogenation or dehydrogenation of chemicalcompounds. For example, the catalysts may be used for convertinghydrocarbons by changing their carbonto-hydrogen ratio at elevatedtemperatures below about 600 C. Such use of a chromic oxide gel catalysthas been described in the United States an improved process LettersPatent No. 1,905,383 issued to W. F.

Huppke and F. E. Frey, April 25, 1933 for a Method of hydrogenating ordehydrogenating hydrocarbons and in an article by Frey and Huppkepublished in Industrial and Engineering Chemistry, vol. 25, January,1933, pages 54 to 59. In its preparation a dilute solution ofa chromicsalt is used and a hydrous precipitate is formed by mixing with analkali solution such as a solution of ammonia or of sodium or potassiumhydroxide. The precipitate is then washed and dried under suitableconditions for obtaining in the end a coherent and catalytically activegel. In certain cases, as has been disclosed in the co-pendingapplication of Frey and Huppke, Serial No. 723,608, filed May 2, 1934for a Process for converting hydrocarbons, now U. S. Patent 2,098,959issued November 16, 1937, it is desirable to incorporate with thechromic oxide one or more difiicultly reducible and difiicultly fusibleoxides, such as alumina, zirconia, thoria. silica, magnesia, titania andboric oxide, in such a way that the gel characteristics are conserved.Heavy metal oxides, such as those of thallium, bismuth, lead and mercurymay also in certain cases contribute desirable qualities to the gelcatalyst.

Heretofore, in the preparation of these catalysts of gel characteristicsand consisting of chromium oxide or containing chromium oxide as aconstituent of the gel, it has been considered necessary that thechromium be in the form of a soluble trivalent chromium salt.Experience, however, has shown time and again that all solutions ofchromic salts do not uniformly lead to the formation of catalysts of thestable gel characteristics which have been found desirable in catalyststo be used, for example, in processes of converting hydrocarbons bychanging their form of a soluble chromic carbon-to-hydrogen ratio atelevated temperatures. For example. when ammonium hydroxide is added toa solution of chromic sulfate, chromic chloride or the violet chromicnitrate, a light blue-green precipitate of chromic hydroxide usuthechemical market not infrequently result in more or less complete failureto yield precipitates which can be dried to make a catalyst of thedesired gel characteristics and catalytic activity. In some cases evencomplete failure to obtain The reasons for these failures are doubtlessquite complex and are not at present completely understood. But it isclear that in consequence of these frequent failures, methods used atpresent are not only at times unreliable but can be inordinatelywasteful of material and energy; for large quantities of catalyst arerequired in the modern commercial scale and failure of a batch ofchemicals to yield the desired precipitate involves a large loss inchemicals and labor.

A further important disadvantage of methods used heretofore is theirfailure to utilize unlformly hexavalent chromi in the form of solu-, blechromates or dichromates, which is much less expensive than trivalentchromium in the salt. The hexavalent chromium in chromates ordichromates must first be reduced to the trivalent condition beforeprecipitation of chromic hydroxide or hydrated chromium oxide can beeffected. After the reduction, which is a step well known in chemicalart, the difiiculties already mentioned may lead to failure to obtainthe desired dark olive green gelatinous precipitate from the resultanttrivalent chromium. Such failures are well-known processes conducted onvalent chromium in chemical literature.

ours in a recent article Vaughen in the Journal A pertinent exampleocpublished by Lazier and of the American Chemical society, vol. 54.,August, 1934, wherein on page 3083 there appears the followingstatement: The reduction of chromic acid with alcohol followed byprecipitation with ammonium hydroxide is a well-known step in theseparation of chromium from certain other elements. When this method wasapplied to the preparation of chromium oxide catalysts, the precipitatesformed were different in character from those obtained under similarconditions from trivalent salts. The precipitates from reduced chromateswere of a chalky blue-green color and had a powdery texture afterwashing and drying, whereas the hydrated oxides prepared from thetrivalent salts were dark green and dried to glossy gel-like masses."

Another marked disadvantage of methods used heretofore for thepreparation of catalysts of gel characteristics and consisting ofchromium oxide or containing chromium oxide as a constituent of the gelis that th presence of the sulfate radical in substantial amount in thesolution of chromic salt, as for example in a solution of chromicsulfate or of chrome alum, invariab y leads to the formation of acatalyst of decidedly inferior catalytic activity for the conversion ofhydrocarbons by changing their carbon-to-hydrogen ratio at elevatedtemperatures. This has in the past largely prevented the economicalrecovery of spent catalysts, which contain the catalytically inactivegreen form of chromic oxide, since this green form of chromic oxide isinsoluble in all ordinary acids but it can be made to dissolve in hotconcentrated sulfuric acid it a small proportion of chromium with avalence greater than three such as the chromium in chromic acid ispresent as a dissolution catalyst.

It is the object of my invention to avoid the dimculties anddisadvantages of the prior art that have been briefly described and toobtain uniformly and at will. catalysts of good and stable gelcharacteristics and consisting of chromium oxide or containing chromiumoxide a a constituent of the gel and active in effecting hydrogenationand dehydrogenation of chemical compounds.

It is a further object of my invention to obtain such catalystsirrespective of the raw material source of. the chromium, whether it bea trivalent chromium salt on the one hand or a hexacompound on theother.

A still further object of my invention is to use the relativelyinexpensive and easily obtained sulfuric acid for the purpose ofdissolving spent catalysts and thus making available the catalyticelements in a form suitable for reworking into a catalytically activegel.

Additional objects and advantages of my invention will be obvious tothose skilled in the art.

I have found that catalysts of the desired gel characteristics andcatalytic activity containing chromium oxide are obtained uniformly andat will if the chromium is first prepared in the form of a colloidal solof chromic oxide in the presence of not more than about one-half as muchof 'a monobasic acid such as for example hydrochloric, or nitric acid,or acetic or other monocarboxylic acid of four or less carbon atoms, aswould be required to convert all of the oxide to the correspondingnormal chromic salt. The chromic oxide used for preparing such acolloidal sol may be in the form of the light blue green aesaeecprecipitate oi chromic oxide which is not infrequently obtained when asolution of an alkali is mixed with a solution of a chromic salt or witha solution prepared by the reduction of a hexavalent chromium compoundto the trivalent condition and which appears very frequently when thesolution contains a substantial proportion of the sulfate radical.Chromic oxide which results from the precipitation of chromic salts byaddition of alkali is more or less hydrous and may vary widely in colorand physical characteristics, sometimes exhibiting the desirable olivegreen color or gelatinous texture to a certain degree. Such precipitatesare in general suitable for making a colloidal sol inmy process.

The manner of using my invention and of obtaining its useful and noveladvantages will be evident from the following detailed description. Aquantity of chromic hydroxide or hydrated oxide is obtained byprecipitation with an alkali from a solution of a trivalent chromiumsalt or from a solution prepared by the reduction of a hexavalentchromium compound to the trivalent condition, irrespective of whether ornot such solution. contains a substantial proportion of the sulfateradical. The hydrated oxide is separated from the solution by filtrationor the equivalent and may be washed with water to reduce the content ofoccluded salts or ions. The hydrated oxide is then treated with about 25per cent as much of a monobasic acid, such as hydrochloric or nitricacid, or acetic or other monocarboxylic acid of four or less carbonatoms, as would be required to convert all of the chromic hydroxide oroxide to the corresponding normal chromic salt, and then the resultingmass is triturated vigorously until it becomes a homogeneous appearingliquid or colloidal sol resembling paint. The water content of themixture is preferably sufficient to produce a buttery to creamyconsistency. The mass is stirred rapidly at perature of about C. untilit becomes very thick and viscous, which may require several hours. Asmall representative sample of the thick viscous sol is then taken anddiluted with hot water and precipitated by the addition of an alkali,advantageously ammonium hydroxide, in slight excess. If the testprecipitate thus obtained is dark olive green in color andverygelatinous in consistency the main body of the sol is diluted withhot water, preferably distilled water, and precipitated with a solutionof an alkali, advantageously with a solution of ammonium hydroxide, andthen conditions are favorable for proceeding with the subsequent washingand drying steps in the preparation of the catalyst. If the testprecipitate does not possess the desired characteristics of a dark olivegreen color and a very gelatinous consistency, a small additional amountof the monobasic acid, suitably one-tenth as much as was used originallyin the preparation of the sol, is added to the main body of the sol andthe stirring and heating is continued for some time longer, after whichanother test precipitate is prepared. It will be found that theprecipitate has become more gelatinous and a darker green in color. Ifit is still not of the most, desirable appearance another small additionof acid is made and the stirring and heating continued as before, andanother test. precipitate prepared. This is continued until the testprecipitate does not show improvement in the desired characteristics ofa dark olive green color and a very gelatinou conhydroxide or hydrated atemaaeasao sistency, or until it has reached a degree of color andgelatinous consistency that are known from experience to be suitable.The final dark olive green and very gelatinous precipitate obtained fromthe main body of the sol will result, after washing and drying undersuitable conditions for obtaining in the end a coherent and stable gel,in a catalyst possessing the desired catalytic properties, such, forexample, as are required to promote the economical and clear-cutdehydrogenation of paraflln hydrocarbons into the corresponding olefinsat elevated temperatures of up to about 600 C.

It is not intended from the foregoing description of my invention thatthe hydrated chromic oxide used for preparing the colloidal sol must befreshly precipitated. I have successfully used hydrated chromic oxidethat had been precipitated many days or weeks before use and I havefound that hydrated chromic oxide that had been dried until it wascomparatively free from moisture is also satisfactory for use by myprocess. In such a case, it may be found to be more eflicacious to useone of the stronger monobasic acids in preparing the colloidal sol.

Other oxides besides chromic oxide may be introduced as constituents ofthe final gel catalyst. This may be accomplished in various ways thatwill be obvious to those skilled in the art.

For example,'they may be incorporated together a with the chromic oxidein the colloidal sol of my process and then coprecipitated with thechromic oxide upon dilution and mixing with an alkaline precipitant.Alternatively, the gelatinous precipitate obtained by adding an alkalinereagent to the chromic oxide sol may be triturated with a hydrous oxideof another species, such as alumina, zirconia, or others, or also with aheavy metal oxide, such as an oxide of thallium, bismuth, lead ormercury, and the oxide mixture dried thereafter to yield the desiredmixed catalyst of gel characteristics.

It is not intended that the proportion of acid to be used in thepreparation of the colloidal sol should be limited to 25 per cent of theamount required to convert all of the chromic oxide to the correspondingnormal chromic salt. I have found that the amount of acid requireddepends somewhat on the character of the chromic oxide and on thepresence of electronegative radicals in the form of salts. It willusually range from to 50 per cent of the chromic hydroxide equivalentbut need not in any case exceed the latter figure. Enough acid may beused to prevent the resultant sol from depositing more than a slightprecipitate upon'dilution with hot water. However, more than a slightprecipitate upon such dilution is tolerated by my process if theprecipitate is of the desired dark olive green color and gelatinousconsistency; but the appearance of appreciable amounts of a light bluegreen and powdery precipitate indicates that additional acid,accompanied by further heating and digesting, is necessary. In general,it will be found well not to use more than half enough acid to reactwith all of the chromic oxide. The water content of the mixture need notbe so low as to produce a viscous mixture or to require trituration toobtain efficient digesting action, but cannot be so high that thechromic hydroxide content of the mixture is less than weight per cent ofthe mixture.

The temperature of about 100 C. used for heating or digesting thecolloidal sol is not critical and somewhat higher or lower temperaturesmay be used, the conversion to the final thick and viscous sol formproceeding more rapidly at the higher temperatures. 'In the final stagesof this step the temperature may be 20 C. or more above C. Still highertemperatures may be used if the sol is heated under pressure in anautoclave. Although this modification of my invention involves the useof rather expensive equipment, it has the advantage that the duration ofthe heating can be thereby materially shortened. It is evident that themost desirable and economical procedure is that which yields the chromicoxide in sol form suitable for the dilution and precipitation steps atthe minimum over-all cost for chemicals, equipment and operatingexpenses.

I wish to make it clear that my invention does not contemplate the useof merely long-continued boiling of a relatively dilute solution of achromic salt accompanied by stepwise addition of an alkaline precipitantsuch as ammonia. Such a procedure gives little or no improvement in thecharacteristics of the precipitate, namely; a dark olive green color, agelatinous consistency and relatively very great specific volume, whichI have found to be necessary for its use in the preparation of asuperior dehydrogenation catalyst such as is uniformly produced by myprocess. For example, one mol, 298 grams, of sodium dichromate,Na2Crz0-1.2H2O, was reduced with sugar in hydrochloric acid solution toform a solution of chromic chloride. The solution was diluted to 20liters and heated to boiling under a reflux condenser which condensedescaping moisture and returned it to the solution. One-half of theamount of ammonium hydroxide required for complete precipitation wasadded and the boiling was continued. Small. quantities of ammoniumhydroxide were added every few hours over a continuous period of fourdays, while the resulting mixture was kept at its boiling temperature,until the solution was basic to litmus. During this period small samplesof the solution were taken at intervals and ammonium hydroxide was addedin slight excess. The resultant precipitate was always a light bluegreen in color and of a non-gelatinous powdery consistency or texture.No improvement or transition to a dark olive green color and agelatinous consistency was observed during the four days boiling of thesolution. The mixture, which now contained all of the chromium asprecipitated hydroxide, was boiled for two more days but the precipitatedid not show any noticeable improvement. The precipitate was washed,filtered and dried in the identical manner used for obtaining goodcatalysts when using a dark, olive green, gelatinous precipitate. Thispreparation, however, showed a very inferior catalytic performance,giving only 5 per cent dehydrogenation of isobutane to isobutylene at450 C. at a fiow rate of 2500 cc. of gas per hour per cc. of catalystand this conversion fell to less than one per cent in about 3 hours. Onthe other hand, a catalyst made from the same reagents and in the samemanner except that the heating and precipitation were made in accordancewith my process, gave a conversion of isobutane to isobutylene of 17 percent for 30 hours under the same conditions of testing.

The dilution of the colloidal sol with hot water prior to mixing with analkali is not critical. Its chief purpose is to render the sol readilymovand any suitable this end may be able, as through plant piping,amount of water to achieve bons.

used. I- have successfully used volumes of water varying from about 1 toover 100 times the volume of the thick viscous sol. Heated water ispreferable to cold water, since the latter will sometimes cause the solto be thrown out of suspension;

Distilled. water may be generally preferable for use in my process butit is not always necessary.

I have made highly satisfactory catalysts when cistern or impoundedrainwater was used and also when pond or lake water was used. However,it is possible that in some geographical localities natural waters maycontain deleterious substances that should be avoided; in such cases,which can be readily determined by trial, distilled water should beused.

The precipitate obtained by adding an alkaline reagent such as ammoniato the diluted s01 yields a coherent and vitreous gel upon drying whichis highly suitable with respect to mechanical properties for use as asolid catalyst. Drying may be efiected partially in air and completedwith the aid of heat. The gel so obtained constitutes a particularlyeflicient catalyst.

The addition to the digested mixture of an alkaline precipitant and thewashing by water prior to drying are essential parts of my procedure andwhile their omission does not prevent the formation of a vitreous gel ondrying, the gel so obtained is an inferior catalyst for hydrogenationand dehydrogenation of hydrocar- By the practice of my invention, I haverepeatedly succeeded in overcoming the difiiculties and disadvantages ofthe prior art. I have been able to obtain uniformly and at willcatalysts of good and stable gel characteristics and consisting ofchromium oxide or containing chromium oxide as a constituent of the geland possessing the desired catalytic activity such as is advantageousfor the conversion of hydrocarbons by changing their carbon-tmhydrogenratio at elevated temperatures. I have obtained repeatedly and withoutexception such catalysts from chromic salts obtained as such in thechemical market or obtained as chromates or dichromates and subsequentlyreduced, and from chromium-containing materials which were known to beincapable of yielding catalysts of the desired characteristics by any ofthe methods used heretofore for this purpose. I have obtained goodcatalysts by my invention from chromic sulfate, from chromic chloride,from the violet chromic nitrate and from chromic solutions containingsubstantial proportions of sulfuric acid or of the sulfate radical, allof which materials constituted in the past unreliable and for the mostpart very unsatisfactory sources of chromium for the desired catalystswhen earlier methods of preparation were used. I have also succeeded inrepeatedly recovering spent catalysts, which contain the green andcatalytically inactive form of chromic oxide by dissolving them in hotconcentrated sulfuric acid in the presence of chromium with a valencegreater than three, such higher valence chromium acting as a dissolutioncatalyst as mentioned previously, and then treating the resultingsulfate solution with alkali to precipitate chromic hydroxide orhydrated oxide and thereafter using the precipitate for the preparationof a colloidal sol of chromic oxide in the manner hereinbeforedescribed. It is advantageous, before efiecting solution of the spentcatalyst in hot concentrated sulfuric acid, to burn off any carbonaceousdeposit that may be on the spent tion has made the production ofcatalysts of gel my invention.

characteristics and containing chromium oxide a reliable. uniformlysuccessful and economical process.

Although I have many times successfully prepared catalysts by thepractice of my invention, I shall limit my descriptions oi specific anddetailed examples to three which indicate a few of the possiblevariations of procedure. It is understood that the details given inthese three examples are illustrative only and are not to be construedas in any way limiting the scope of Example I In my first example onemol, 298 grams, of sodium dichromate, NazCrsOrZHaO, was reduced withsugar in the presence of an excess of suliuric acid. The resultingchromic sulfate solution was diluted with about 10 liters of hot waterand ammonium hydroxide in slight excess was added to the hot solution toprecipitate the chromium. The resultant precipitate of hydrated chromicoxide was of a light blue green color and of a powdery or chalk-likeconsistency. It was washed by decantation, filtered and pressed toremove excess water. then triturated in about 410 cc. of hydrochloricacid of specific gravity 1.19 until no lumps remained. The resultanthomogeneous-appearing mixture was heated with rapid stirring at about C.or more for about an hour. A small amount of the sol, which was nowquite thick and viscous, was then dilutedwith hot water and precipitatedwith ammonium hydroxide. Since the precipitate did not have the desiredcharacteristics of a dark olive green color and of a gelatinousconsistency, 25 co. more of hydrochloric acid of the same strength wasadded to the main body of the sol and the stirring and heating wascontinued for about three additional hours. A second test precipitatethen showed the desired dark olive green color and gelatinousconsistency. Accordingly, the main body of the sol was diluted with 10liters of hot water and hydrated chromic oxide was precipitated by theaddition of aqueous ammonia in slight excess. After filtering, washingand pressing to remove excess water, the dark olive green and gelatinousprecipitate of hydrated chromic oxide was dried to a hard, black,glossy, vitreous gel which was catalytically very active, rugged andlong-lived for the conversion of isobutane to isobutylene.

Example II A second specific detailed example of the use of my processis as follows. One mol, 298 grams, of sodium dichromate, Na2Cr2O7-2H2O,was reduced with sugar in the presence of hydrochloric acid.Three-fourths of the resulting chromic chloride solution was diluted to20 liters with hot water and ammonia in slight excess was added to thehot solution to precipitate the chromium The precipitate was idly over aboiling water bath for one hour. A sponding chromic salts and may beregarded as small amount of the sol was then diluted with hot water andprecipitated with ammonium hydroxide. The precipitate did not possessthe desired characteristics of a dark olive green color and a gelatinousconsistency. Accordingly, cc. of concentrated hydrochloric acid wasadded to the sol and the stirring was continued over a hot glycerinebath for some time. A second test precipitate then showed considerableimprovement, and 10 cc. more of concentrated hydrochloric acid was addedand the stirring and heating was continued. After some time a third testprecipitate was obtained. Since this third test precipitate possessedthe desired dark olive green color and gelatinous consistency, the mainbody of the sol was diluted with about liters of hot water and ammoniumhydroxide was added in slight excess to precipitate the chromium ashydrated chromic oxide. The precipitate was filtered, washed and pressedto remove excess water. Upon drying, the precipitated hydrated chromicoxide became a hard, black, glossy, vitreous gel which was catalyticallyvery active, rugged and long-lived when tested with isobutane forconversion of isobutane to isobutylene at elevated temperatures up toabout 550 C.

Example III In my third specific example, 400 grams of violetcrystalline chromic nitrate, Cr(N0a)a.9mO. were melted by heating. Then140 cc. of 28 per cent aqua ammonia were added stepwise while themixture was stirred and heated. Continued heating and evaporation causedthe light blue chalky precipitate first formed to become dispersed toform a homogeneous-appearing sol and further heating and evaporationmade the sol very thick and viscous. After three hours of heating andstirring during which the temperature increased to 122 C., a portiondiluted with hot water and treated with a slight excess of ammoniumhydroxide yielded a dark olive green and gelatinous precipitate.Accordingly, the main body of the thick sol was diluted with about 10liters of hot water and hydrated chromic oxide was precipitated by theaddition'of aqua ammonia in slight excess. The hydrated chromic oxidewas washed three times by decantation, fitered, pressed to remove excesswater, and dried to a hard black glossy vitreous gel. The final driedgel possessed excellent catalytic activity for the conversion of gaseousparafins to oleiins and was rugged and long-lived.

I do not wish to exclude from my invention certain modifications oralternatives which are obvious to those skilled in the art. For example,instead of a monobasic acid mentioned hereinbefore in connection withthe preparation of the sol of chromic oxide, it is obvious from thespecific examples cited that an equivalent amount of the correspondingchromic salt can be employed, since neutralization and hydrolysisreactions proceed reversibly to produce in either case free acid andchromic salt. Likewise in the preparation of mixed catalysts, the acidmay be introduced in a chemically combined state with one or moreof themetallic elements of the catalyst other than chromium, as in the form ofone or more salts. Chromic salts of monobasic acids have thecharacteristic that they are hydrolyzed J in aqueous solution to givethe free acid, and f other metallic salts of monobasic acids, whichpossess this characteristic may be used in place of the monobasic acidsor in place of the correamount of such salt not equivalents in thediscussion or claims. 01. course, it is obvious that the salts of metalswhich would impair the catalytic activity of the final gel should not beused. Furthermore, I do not wish to limit my invention to the use ofonly the monobasic acids specifically mentioned hereinbefore namely,hydrochloric, nitric, and acetic acids, for thesultability of an acidfor use in my process can be readily determined by trial. Hence, Idesire to have it understood that, within the scope of the appendedclaims, my invention is as extensive in scope and equivalents as theprior art allows.

I claim: 1 l. A process for preparing a catalyst 01 gel characteristicsand containing chromium oxide as a constituent of the gel comprisingpreparing an aqueous colloidal sol by mixing chromic oxide and aconcentrated aqueous solution of a monobasic acid whose normal chromicsalt is soluble in water and hydrolyzes to give free acid insuflicientin amount to convert more than per cent of the chromic oxide into thecorresponding normal salt, digesting said sol at a temperature of about100 C. mixing the colloidal so] with a solution of an alkali in slightexcess, precipitating dark gelatinous chromium oxide and washing anddrying the resultant precipitate.

2. A process of preparing catalysts of gel characteristics andcontaining chromium oxide as a constituent of the gel comprisingagitatin hydrated chromic oxide comprised of blue, chalky chromium oxidewith a hot aqueous solution of a chromic salt of a monobasic acid whosenormal chromic salt is soluble in water and hydrolyzes to give free acidto produce a colloidal solution, the

exceeding that which contains monobasic acid radicals chemicallyequivalent to the hydrated chromic oxide, heating and digesting thecolloidal solution, diluting the solution with hot water, mixing thediluted solution with a slight excess of a solution of an alkali, andwashing and drying the resulting dark, gelatinous precipitate.

3. In a process of preparing catalysts of gel characteristics andcontaining chromium oxide as a constituent of the gel, the steps whichcomprise heating a precipitated hydrous chromic oxide in the blue,chalky form, intermixed with not more than one half as much of amonobasic acid whose normal chromic salt is soluble in water andhydrolyzes to give free acid as would be resuch a time that a thickviscous colloidal sol of chromium oxide is obtained and digested,diluting said sol with at least an equal volume of hot water, adding tothe dilute solution alkali in slight excess and precipitatin chromiumoxide as a dark, gelatinous precipitate.

4. In a. process of preparing chromium oxide gel, the step whichcomprises digesting a mixture comprising bluish, chalky hydrous chromicoxide and a concentrated acidic aqueous medium whose anions consistsubstantially entirely of anions of a monobasic acid whose normalchromic salt is soluble in water and hydrolyzes to give free acid, saidmixture containing said anions in such an amount, not to exceed onehalfof the chemical equivalent of the total chromium present figured astrivalent chromium, and said digesting being continued for such apecapable of yielding a dark green and gelatinous an alkali in slightexprecipitate on addition of cess, and subsequently adding a solution oran alkali in slight excess.

5. A processior the preparation of a catalyst of gel characteristics andcontaining chromium oxideas a constituent ofthe gel, which comprisesdigesting a mixture comprising bluish ing a dark green and gelatinousprecipitate on subsequent addition of an alkali in slight excess, mixingsaid sol with a solution of an slight excess, thereby precipitating darkgreen and gelatinous hydrous chromic oxide, and washing and drying theresultant precipitate.

6. A process for the preparation of a catalyst of gel characteristicsand containing chromium oxide as a constituent of the gel, whichcomprises digesting at about 100 C. a mixture comprising bluish chalkyhydrous chromic oxide and a concentrated acidic aqueous medium whoseanions consist substantially entirely of anions of a monobasic acidwhose normal chromic salt is soluble in water and hydrolyzes to givefree acid, said mixture containing said anions in such an amount, not toexceed one-half of the chemical equivalent of the total chromium presentfigured as trivalent chromium, and said digesting being continued forsuch a period that a thick viscous sol is obtained that is capable ofyielding a dark green and gelatinous precipitate on subsequent additionof an alkali in slight excess, diluting said sol with at least an equalvolume of hot water, mingling said diluted sol with a solution of analkali in slight excess, thereby precipitating dark green and gelatinoushydrous chromic oxide, and washing and drying the resultant precipitate.

'7. The process oiclaim in which the said bluish chalky hydrous chromicoxide has been obtained by recovery from a spent catalyst containingchromium oxide by dissolution of the spent catalyst in hot concentratedsulfuric acid in the presence of a small amount of chromium having avalence greater than three and precipitation from the resultant sulfuricacid solution by an alkali in slight excess. v

8. The process of claim 5 in which the alkali is ammonium hydroxide.

9. The process of claim 6 in which the said acidic aqueous medium is aconcentrated solution of a monobasic acid.

10. The process of claim 6 in which the said acidic aqueous medium is aconcentrated solution of a monobasic acid selected from the groupconsisting of hydrochloric, nitric, and monocarboxylic acid of not morethan four carbon atoms per molecule. A

11. The process of claim 5 in which the said acidic aqueous medium isconcentrated hydrochloric acid.

12. The process of claim .5 in which the said acidic aqueous medium isconcentrated nitric acid.

13. The process of claim 6 in which the said acidic aqueous medium is aconcentrated solution of a chromic salt. a

alkali in aaeaseo 14. The process of claim 5 in which the said acidicaqueous medium is a concentrated solution of a chromic salt from whichabout two thirds of the chromium has been precipitated as the hydrouschromic oxide of said mixture.

15. A process for the preparation of a catalyst of gel characteristicsand containing chromium oxide as a constituent of the gel, whichcomprises digesting at about 100 C. a mixture comprising bluish chalkyhydrous chromic oxide and about one-fourth as much of a concentratedaqueous solution of a monobasic acid whose normal chromic salt issoluble in water and hydrolyzes to give free acid as would be requiredto convert all of the chromic oxide to the corresponding normal chromicsalt until the mixture becomes a very thick and viscous sol, adding fromtime to time additional smaller amounts.

of the monobasic acid while continuing the digesting until a small testportion of the thick viscous sol, when diluted with hot water andtreated with a solution of an alkali, yields a dark green and gelatinousprecipitate, the total amount of monobasic acid not exceeding onehalfthat required to convert all of the chromic oxide to normal chromicsalt, then diluting the main portion of the thick viscous sol with hotwater, mixing the diluted sol with an alkali in slight excess, therebyprecipitating dark green and gelatinous hydrous chromic oxide, andwashing and drying the resultant precipitate.

16. A process for the preparation of a catalyst of gel characteristicsand containing chromium oxide as a constituent of the gel, whichcomprises digesting at about 100 C. a mixture comprising bluish chalkyhydrous chromic oxide and a concentrated aqueous solution of a chromicsalt of a monobasic acid, in-an amount contain ing about one-half asmuch chromium as said chromic oxide, until the mixture becomes a verythick and viscous sol, adding from time to time additional small amountsof the chromic salt while continuing the digesting until a small testportion of the thick viscous sol, when diluted with hot water andtreated with a solution of an alkali, yields a dark green and gelatinousprecipitate, the chromium in the total amount or chromic salt added notexceeding the total amount of chromium in the said hydrous chromicoxide, then diluting the main portion of the thick viscous sol with hotwater, mixing the diluted sol with an alkali in slight excess, therebyprecipitating dark green and gelatinous hydrous chromic oxide, andwashing and drying the resultant precipitate.

17. A process of preparing a chromium oxide gel, which comprisesdigesting a mixture comprising bluish chalky hydrous chromic oxide in aconcentrated acidic aqueous medium whose anions consist substantiallyentirely of anions o! a monobasic acid whose normal chromic salt issoluble in water and hydrolyzes to "give free acid, said mixture beingacidic, to. an extent-riot greater than that substantially equivalent toamixture of chromic oxide and one-half its chemical equivalent og such amonobasic acid, said digesting being tiontinued for such a period oftime that a sol is obtained that is capable of yielding a dark green andgelatinous precipitate on the subsequent addition of an alkali in slightexcess, diluting said sol with hotwater, adding an alkali in slightexcess to precipitate dark green and gelatinous hydrous chromic oxide,and washing and drying the resultant precipitate.

about 100 C. a mixture comprising bluish chalky hydrous chromic oxideand a concentrated aqueous solution of a soluble salt of a monobasicacid, the metallic element of said salt being a member of the groupconsisting of aluminum, zirconium, titanium, silicon, thorium, boron,and magnesium, in an amount not to exceed about one-half as much of saidsalt as is equivalent to said chromic oxide, until the mixture becomes athick viscous sol that is capable of yielding a dark green andgelatinous precipitate on subsequent addition of an alkali in slightexcess, mixing said sol and a solution of an alkali in slight excess toprecipitate dark green and gelatinous hydrous chromic oxide along with agelatinous hydrous oxide of said metal, and drying the precipitate toform a dried gel.

19. In the preparation of a chromium oxide gel catalyst the stepscomprising adding concentrated ammonium hydroxide to a concentratedlution, and drying the precipitate to form a black vitreous chromiumoxide catalyst.

20. The process of claim 5 in which said acidic aqueous medium is aconcentrated solution of a. salt containing a desired metallicconstituent of said catalyst, other than chromium, and possessing thecharacteristic of hydrolyzing in aqueous solution to give the free acid.

21. The process of claim 19 in which said con centrated solution ofchromic nitrate is prepared by melting chromic nitrate nonahydrate. V

22. The process of claim 5 in which said acidic aqueous medium is aconcentrated solution of an aluminum salt of a monobasic acid.

23. The process of claim 5 in which said acidic aqueous medium is aconcentrated solution of a. zirconium salt of a monobasic acid.

GLEN H. MOREY.

